This invention relates to an improved method for the fluid catalytic cracking of hydrocarbon oils. In particular, this invention relates to the fluid catalytic cracking of petroleum feed stocks. More particularly, it is related to a method of operating a fluid catalytic cracking unit (FCCU) having a multiplicity of elongated reaction zones, also described herein as risers, in order to obtain higher yields of naphtha at higher octane ratings than obtained heretofore.
In the fluid catalytic cracking process, hydrocarbons are converted under conditions such that substantial portions of a hydrocarbon feed are transformed into desirable products such as gaoline, liquified petroleum gas, alkylation feed stocks and middle distillate blending stocks with concomitant by-product formation of an undesirable nature, such as, gas and coke. When substantial amounts of coke deposition occur, reduction in catalyst activity and, particularly, catalyst selectivity results thereby detering hydrocarbon conversion, reducing gasoline production and simultaneously increasing the production of less desired products. To overcome such catalyst deactivation through coke deposition, the catalyst is normally withdrawn from the reaction zone and passed to a stripping zone wherein entrained and adsorbed hydrocarbons are initially displaced from the catalyst by means of a stripping medium such as steam. The steam and hydrocarbons are removed and the stripped catalyst is passed to a regeneration zone where it is contacted with an oxygen-containing gas to effect combustion of at least a portion of the coke and regeneration of the catalyst. Thereafter, the regenerated catalyst is reintroduced to the reaction zone and therein contacted with additional hydrocarbons.
Recently there have been significant improvements in the catalytic cracking process. The introduction of zeolitic cracking catalysts has resulted in increased throughput and improved product quality from existing catalytic cracking units. In addition, improved catalytic cracking apparatus has been developed specifically for use with these improved catalysts which has further enhanced the overall process. For example, U.S. Pat. Nos. 3,433,733 and 3,448,037 are directed to the fluid catalytic cracking of petroleum hydrocarbons with zeolitic cracking catalysts and disclose operating parameters and techniques particularly directed to these new catalysts, as well as fluid catalytic cracking apparatus designed to take advantage of the desirable characteristics of the zeolitic catalysts. More particularly, the apparatus incorporates the concept of "riser cracking" wherein gas oil feedstocks and cycle stocks are individually cracked in separate elongated reaction zones or risers terminating in a tapered reaction chamber containing a dense phase and a dilute phase of catalyst wherein further cracking takes place in the fluidized dense phase. With riser cracking it is possible to select operating conditions specifically suited to the particular feed, gas oil or recycle stocks, being introduced into each riser. In addition, the cracking apparatus incorporates a stripping section beneath the fluidized dense bed wherein entrained and adsorbed hydrocarbons are displaced from the catalyst by means of steam before the catalyst passes to the regenerator vessel. As with conventional fluid catalytic equipment the catalyst is contacted with an oxygen-containing gas in the regenerator to effect combustion of at least a portion of the deposited coke. The regenerated catalyst is then reintroduced into the bottom of the elongated reaction zones or risers at a point where the feedstocks are introduced. Through the use of multiple risers it is possible to operate the individual risers under conditions providing maximum recovery of desired products with minimal formation of undesired materials at the maximum possible yields. Usually, the more refractory stocks, such as the cycle gas oils, will be passed through the riser operating at a higher temperature than the riser which is processing the fresh gas oil. The overall effect, of course, is to achieve optimum performance of both the fluid catalytic cracking unit and the zeolitic cracking catalyst.
Although the prior art patents have disclosed apparatus and operating procedures which take advantage of the unique properties of the zeolitic cracking catalyst, any other operating improvements which will further enchance the performance of the apparatus or the quality and/or quantity of the products are highly desirable.